Programmable controllers are typically connected to industrial equipment, such as assembly lines and machine tools. These special purpose computers sequentially operate the equipment according to a stored program. One such programmable controller is disclosed in U.S. Pat. No. 4,413,319. This type of controller includes an I/O scanner module that receives input data from numerous remote sensors via a common serial data link.
Originally the remote sensors were relatively simple devices, such as limit switches and other devices which produced a binary output that resembled a switch. As the need developed for more complex sensing functions, devices, such as temperature sensors, emerged which produce multiple bits of data representing the sensed conditions. These sensors produce blocks of data which have to be sent over the data link to the programmable controller for processing. Therefore, a communication system, such as taught by U.S. Pat. No. 4,413,319, was developed to transfer the blocks of data from the remote sensors.
As the industrial control technology evolved even further, the sensors became more sophisticated becoming programmable devices themselves. One type of these devices is radio frequency (RF) tag antennas, such as the Allen-Bradley Bulletin 2750 series. These "antennas"are transceivers which interrogate transponder tags fastened to objects, such as automobiles, as the objects move past the antenna on an assembly line, for example. Not only is the antenna programmable, but certain types of tags can be programmed with identification data by one antenna and interrogated to transmit the data to another antenna farther down the assembly line.
Quite often the process of obtaining data from one of these sophisticated sensors involved the programmable controller sending several operation commands to the sensor via the data link. For example, if a tag is to be programmed by an antenna, the first operation could be to notify the programmable controller when a tag is detected in the range of the antenna. The next operation would be to program the tag with data that accompanies the programming command. The final operation would be to read the tag and return the data to the programmable controller for verification that the tag was properly programmed. In conventional sensor networks, this procedure requires that three separate messages and three responses be sent over the data link.
Typically the sensor devices are not connected directly to the data link. Rather, an interface circuit is employed which often couples several sensor devices to the data link. The interface circuit handles the communication over the link and is programmed to translate the data message protocols of the data link and sensor devices. Therefore, a variety of sensor devices can be used and the sensor devices do not have to be designed for use with that particular data link protocol. Another complexity of conventional sensor networks is the use of different protocols for messages sent by the controller to either the interface circuits or to the sensor devices.